"I’m hitting the trail to earn your vote. Headed to Iowa. –H http://hrc.io/running," she subsequently tweeted, following that with a Spanish-language version of the original announcement.

Clinton fans for the Progressive Change Campaign released a video timed for the announcement, urging her to "go big" with her ideas.

"Hillary Clinton’s campaign launch begins an important stage of the national conversation," said campaign cofounder Adam Green. "On most issues of concern to voters, the debate inside the Clinton campaign and across the nation will not be about going left versus right, but rather going big versus small. We hope Hillary Clinton thinks big and takes on powerful interests on behalf of everyday working families. Americans need Clinton to focus the national conversation on big, bold, economic populist ideas – like debt-free college, expanding Social Security, clean-energy jobs, Wall Street reform, campaign finance reform, and helping workers share in the wealth they create."

On Meet the Press Sunday, former campaign manager and current New York mayor Bill de Blasio said he was not endorsing her until he saw "an actual vision of where she wanted to go."

But he also said she would be one of the most qualified people ever to run for the office. "She doesn't need to be vetted," he said, "but she has to address the issues.”

Not everyone welcomed the announcement.

The California Republican Party immediately released a statement: “Hillary Clinton’s actions have proven that she is not the leader America needs,” said party chairman Jim Brulte, though he suggested she would be getting support from Hollywood's "limousine liberals."

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